In the inventions described in the aforementioned related patent applications, an environmentally safe, non-polluting diluent that can be used to thin very highly viscous polymer and coating compositions to liquid spray application consistency is discussed. The diluent utilized, as discussed in these aforementioned related applications, is a supercritical fluid, such as supercritical carbon dioxide and nitrous oxide.
U.S. Pat. No. 4,923,720 discloses processes and apparatus for the liquid spray application of coatings to a substrate that minimize the use of environmentally undesirable organic diluents. The broadest process embodiment of that application involves forming a liquid mixture in a closed system, said liquid mixture comprising at least one polymeric compound capable of forming a coating on a substrate and at least one supercritical fluid in at least an amount which when added to the liquid mixture is sufficient to render the viscosity of the mixture to a point suitable for spray application; and spraying said liquid mixture onto a substrate to form a liquid coating thereon. The application is also directed to a liquid spray process in which the preferred supercritical fluid is carbon dioxide. The process employs an apparatus, which among other things, has the means for supplying supercritical carbon dioxide fluid.
U.S. application Ser. No. 413,517 is directed to methods and apparatus for effectively proportionating a mixture of compressible and non-compressible fluids and, in particular, to the formation of a coating composition mixture containing a substantially accurate proportionated amount of at least one supercritical fluid used as a viscosity reduction diluent.
Because of its relevancy to the present invention, a brief discussion of supercritical fluid phenomena is believed to be warranted.
Supercritical fluid phenomena is well documented, see pages F-62 - F-64 of the CRC Handbook of Chemistry and Physics, 67th Edition, 1986-1987, published by the CRC Press, Inc., Boca Raton, Fla. At high pressures above the critical point, the resulting supercritical fluid, or "dense gas" will attain densities approaching those of a liquid and will assume some of the properties of a liquid. These properties are dependent upon the fluid composition, temperature, and pressure. As used herein, the "critical point" is the transition point at which the liquid and gaseous states of a substance merge into each other and represents the combination of the critical temperature and critical pressure for a given substance. The "critical temperature", as used herein, is defined as the temperature above which a gas cannot be liquified by an increase in pressure. The "critical pressure", as used herein, is defined as that pressure which is just sufficient to cause the appearance of two phases at the critical temperature.
The compressibility of supercritical fluids is great just above the critical temperature where small changes in pressure result in large changes in the specific volume of the supercritical fluid. The "liquid-like" behavior of a supercritical fluid at higher pressures results in greatly enhanced solubilizing capabilities with higher diffusion coefficients and an extended useful temperature range compared to liquids. Compounds of high molecular weight can often be dissolved in the supercritical fluid at relatively low temperatures.
Near-supercritical liquids also demonstrate solubility characteristics and other pertinent properties, such as high compressibility, similar to those of supercritical fluids. These variations are considered to be within the concept of a supercritical fluid as used in the context of this invention. Therefore, as used herein, the phrase "supercritical fluid" denotes a compound above, at, or slightly below the critical temperature and critical pressure (the critical point) of that compound.
In essentially every process in which a mixture is prepared for a particular purpose, the constituents of that mixture usually need to be present in particular, proportionated amounts in order for the mixture to be effective for its intended use. In the aforementioned related patent, the underlying objective is to reduce the amount of organic solvent present in a coating formulation by use of supercritical fluid. Therefore, it is particularly preferred that there be prescribed, proportionated amounts of supercritical fluid and of coating formulation present in the liquid coating mixture to be sprayed.
Accordingly, in order to spray liquid coating formulations containing supercritical fluid as a diluent on a continuous, semi-continuous, and/or an intermittent or periodic on-demand basis, it is necessary to prepare such liquid coating formulations in response to such spraying by accurately mixing a proportioned amount of the coating formulation with the supercritical fluid. However, the compressibility of supercritical fluids is much greater than that of liquids. Consequently, a small change in pressure results in large changes in the specific volume of the supercritical fluid. Liquids also become highly compressible as the temperature and pressure approach critical conditions and therefore liquid properties and gas properties approach each other. As an example, FIG. 1 shows the specific volume-pressure relationship for liquid carbon dioxide (Quinn, E. L. and Jones, C. J., Carbon Dioxide, Reinhold, 1936). It can be seen that near or around the critical region, denoted as B in the figure, the specific volume-pressure isotherms become more steeply sloped. The compressibility is the slope of the isotherm, that is, the change in specific volume that occurs with change in pressure at constant temperature. The curve denoted as C, which runs from A to B, is the liquid-vapor equilibrium curve. The liquid compressibility at a pressure of about 1100 psi is given below for several temperatures.
-30.degree. C. 0.2 percent/100-psi PA1 0.degree. C. 0.5 percent/100-psi PA1 10.degree. C. 0.9 percent/100-psi PA1 20.degree. C. 1.8 percent/100-psi PA1 25.degree. C. 3.3 percent/100-psi PA1 30.degree. C. &gt;13.5 percent/100-psi PA1 -30.degree. C. 1.5 percent PA1 -20.degree. C. 2.5 percent PA1 -10.degree. C. 3.6 percent PA1 0.degree. C. 4.9 percent PA1 10.degree. C. 6.6 percent PA1 20.degree. C. 9.0 percent PA1 25.degree. C. 13.0 percent PA1 30.degree. C. 23.0 percent
The overall percentage change in liquid volume that occurs when liquid carbon dioxide is pressurized at constant temperature from its vapor pressure (curve C) to a pressure of 1400 psi, which is a typical pressure at which coating materials are sprayed with supercritical carbon dioxide, are given below for several temperatures.
This shows that the compressibility increases by about an order-of-magnitude as the temperature increases from refrigerated temperatures to room temperature, which is typically close to the critical temperature of 31.degree. C. Even at the lower pressures, the specific volume change with pressure (liquid compressibility) is significant enough to make the pumping of accurate volumes, per the volumetric displacement in a piston pump, for example, difficult.
The compressibility of the supercritical fluids causes the flow of these materials, through a conduit and/or pump, to oscillate or fluctuate. As a result, when mixed with the coating formulation, the proportion of supercritical fluid in the resulting admixed coating formulation also correspondingly oscillates or fluctuates instead of being uniform and constant. Moreover, the compressibility of liquid carbon dioxide at ambient temperature is high enough to cause flow oscillations and fluctuations to occur when using reciprocating pumps to pump and proportion the carbon dioxide with the coating formulation to form the admixed coating formulation. This particularly occurs when the volume of liquid carbon dioxide in the flow path between the pump and the mixing point with the coating formulation is too large. The oscillation can be promoted or accentuated by any pressure variation that occurs during the reciprocating pump cycle.
In an embodiment discussed in the aforementioned related patent, an apparatus is disclosed for pumping and proportionating a non-compressible fluid with compressible carbon dioxide fluid in order to prepare the ultimate mixture to be sprayed with the carbon dioxide in its supercritical state. In that embodiment, volumetric proportionating of the coating formulation stream and the liquid carbon dioxide stream is carried out by means of reciprocating pumps, which displace a volume of fluid from the pump during each one of its pumping cycles. One reciprocating pump is used to pump the coating formulation and it is slaved to another reciprocating pump that is used to pump the liquid carbon dioxide. The piston rods for each pump are attached to opposite ends of a shaft that pivots up and down on a center fulcrum. The volume ratio is varied by sliding one pump along the shaft, which changes the stroke length.
However, as aforementioned, even when stored at ambient temperature, liquid carbon dioxide is relatively compressible. Such compressibility may undesirably cause fluctuation in the amount of carbon dioxide that is present in the admixed coating formulation that is to be sprayed. This occurs due to the incompatible pumping characteristics of the relatively non compressible coating formulation and the relatively compressible liquid carbon dioxide. With the coating formulation, pressure is immediately generated in the reciprocating pump as soon as its volume is displaced. Inasmuch as the liquid carbon dioxide is substantially compressible, a larger volume is needed to be displaced in order to generate the same pressure. Because mixing occurs when the flow of the coating formulation and of the liquid carbon dioxide are at the same pressure, the flow rate of carbon dioxide lags behind the flow rate of the coating formulation.
This oscillation is accentuated if the driving force operating the pump varies during the operating cycle, such as an air motor changing direction during its cycle. Thus, if the driving force declines, the pressure in the coating formulation flow declines even more rapidly, due to its non-compressibility, than the pressure in the liquid carbon dioxide flow, due to its being compressible.
Accordingly, the pressures generated in both flows may be out of phase during the pumping cycle, such that the proportion of carbon dioxide in the mixture to be sprayed also varies. This oscillation is made even more severe if cavitation also occurs in the carbon dioxide pump due to vapor formation as the pump fills.
Resolution of inaccuracy in the proportionation ascribed to fluctuation in the flow of the compressible fluid is discussed in the aforementioned related U.S. patent application Ser. No. 413,517, wherein methods and apparatus are disclosed for accurately and continuously providing a proportionated mixture comprised of a non-compressible fluid and a compressible fluid for spraying onto a substrate to be coated. In particular, mass proportionation is relied upon to obtain the desired mixture of the compressible and non-compressible fluids. Specifically, the mass flow rate of the compressible fluid is continuously and instantaneously measured. Regardless of what that flow rate is and whether or not it is oscillating as a result of, for example, being pumped by a reciprocating pump or regardless of the state in which such compressible fluid is in, that mass flow rate information is fed to a signal processor in a continuous and instantaneous manner. Based on that received information, the signal processor, in response to the mass of compressible fluid that has been measured, controls a metering device which controls the rate of flow of the non-compressible fluid. The non-compressible fluid is then metered in a precise, predetermined proportion relative to the mass flow rate of the compressible fluid such that, when the compressible and non-compressible fluids are subsequently mixed, they are present in the admixed coating formulation in the proper proportions.
By measuring the mass flow rate of the substantially compressible fluid, and then controlling the amount of non-compressible fluid in response thereto, the measuring fluctuation problem associated with the compressibility of the compressible fluid is substantially eliminated. Any fluctuations or oscillations present in the flow of the compressible fluid are instantaneously measured and are compensated by controlling the amount of non-compressible fluid to provide the prescribed proportionation for the desired mixture. In contrast to past techniques, the embodiment in the said patent application involves the metering, i.e., controlling the flow rate, of only one fluid, namely, the non-compressible fluid. The flow rate of the compressible fluid is not controlled, but rather, only measured, from which measurement the prescribed amount of non-compressible fluid is correspondingly adjusted to provide the desired proportionation. This allows for total flexibility of the system and provides for a simple and effective means for producing the desired proportionated mixture of compressible and non-compressible fluids.
While the measuring aspect of the fluctuation problem for obtaining a proper mass ratio of an admixture of compressible and non-compressible fluids, such as admixture of coating formulation and carbon dioxide, has essentially been solved by the foregoing invention using mass proportionation, it has not been solved where such mass proportionation is not utilized and cavitation occurs in pumps and other apparatus supplying the compressible fluid such as liquid carbon dioxide.
Cavitation is a phenomenon that can cause severe equipment damage. In pumps, for example, in addition to the potential of causing damage to the pump itself, the efficiency of such pumping also may suffer severely. Without wishing to be bound by theory, cavitation is the formation and collapse of vapor cavities in a flowing liquid. In a flowing liquid, if at any time the pressure reduces to that of the vapor pressure of the liquid at the temperature of such flowing liquid, the liquid boils and small bubbles, or cavities, of vapor form in large numbers. These bubbles are present in the flow and upon reaching a point where the pressure is higher, they collapse suddenly as vapor condensation occurs, such as in a pump. With condensation, a void is created and the surrounding liquid rapidly fills it, with collision occurring in the center of the void. This results in the creation of very high local pressures, which have been measured at pressures up to 200,000 pounds per square inch. All surfaces in contact with the liquid are subject to the consequences of this phenomenon, due to the energy being transmitted by these pressure waves. The equipment can be damaged, or fail, by fatigue and/or erosion, which is aided by corrosion with the surface becoming badly scored and pitted. Mechanical breakdown of equipment components can also occur due to metal removal, for example. Cavitation may also be attended by considerable vibration and noise.
Another significant problem associated with cavitation in a pump is its effect on the pump's capacity. Cavitation creates much more heat from the mechanical work of compression, which then starts a feedback cycle that may cause the pump to become inoperative. Thus, cavitation results in the generation of heat in the pump. This makes the contents of the pump more compressible which results in the further heating of the compressible fluid being pumped at the inlet to the pump. This, in turn, results in escalating the cavitation, which in turn results in yet further heating and even greater cavitation. Accordingly, to keep the pump operationally stable, it is extremely desirable that cavitation be controlled by some technique that limits the internal heating caused by the work of compression.
Protection against cavitation damage may take several forms such as, for example, the use of highly resistant materials where cavitation is expected, which may include special coatings, welded overlays, and sprayed materials; cathodic protection; and hydrodynamic design.
With respect to cavitation-inhibiting and corrosion-inhibiting compounds, U.S. Pat. No. 4,404,113, issued Sep. 13, 1983, discusses the use of many suitable compounds such as glycols, alkali metal tetraborates, mercaptans, sulfated ethers, alkali metal silicates, aliphatic higher alcohols , cellulose ethers, polyvinylpyrroliodone, polyhydric alcohols selected from the group comprising glycerol alkylene glycols having 2 to 6 carbon atoms, and oxyalkylene glycols of oxyethylene and/or oxypropylene, having a total of 4 to 12 carbon atoms, as the main constituent. However, the presence of such compounds in materials to be pumped may not always be tolerable. For example, in the aforementioned related patent, such components would cause contamination of the coating admixture which would lead to, among other things, imperfections in the coating sprayed onto the substrate.
Protection against cavitation by hydrodynamic design includes, but is not limited to, consideration of proper size of conduits and fittings to minimize pressure loss due to frictional effects during the flow of the liquid, especially when using a high viscosity fluid; the mechanical design of pumps to minimize the potential of internal pressure changes that are conducive to cavitation; the location of the liquid source and the pump suction line; and maintaining the required "net positive suction pressure", which is defined as the absolute pressure above the vapor pressure of the liquid at the pump inlet that is required to prevent the phenomena caused by cavitation. Therefore, control of operating temperature, thereby controlling the vapor pressure of the liquid, and controlling the pump's upstream and downstream pressures, without affecting the desired pressure increase, are effective in the essential relief of cavitation.
In the pumping of cryogenic liquids and/or liquified compressed gases, reciprocating-type pumps are most often employed, although rotary vane pumps are also used for pumping cryogenic liquids. With reciprocating-type pumps, one area of concern in pumping is the problem associated with heat management. Heat conduction from the warm end of the pump to the pumping chamber portion of the pump body, heat leakage from the ambient environment, frictional heat generated by the reciprocating motion of the plunger, and heat generation in the pumping chamber due to fluid compression are recognized as major sources of pump inefficiency. As discussed by Pevzner in U.S. Pat. No. 4,576,557, the principal approach for overcoming such problems has been to intercept the heat conducted from the warm end of the pump by means of heat exchange with a cold fluid. Thus, pumps utilizing suction liquid, blowby fluid, and pressurized liquid have been proposed in the art. For example, Picard, U.S. Pat. No. 1,895,295, describes the submersion of the pump in a cryogenic liquid and the use of heat transfer fins on the pump body to improve heat transfer between the pumping chamber and the pumped liquid. Similarly, Hughes, U.S. Pat. No. 2,931,313, and Lady, U.S. Pat. No. 2,973,629, provide an annular cooling jacket surrounding the pumping chamber, with cryogenic liquid being passed through said cooling jacket prior to being introduced into the pumping chamber on the suction stroke of the pump. In Riede, U.S. Pat. No. 2,730,957, Gottzmann, U.S. Pat. No. 3,136,136, and Schuck, U.S. Pat. No. 4,156,584, blowby fluid is passed in a direction opposite to the heat flux so as to intercept the heat conducted from the warm end of the pump. These approaches can effectively prevent major problems, such as vapor binding, which would normally accompany an inordinate heat flux to the cold end of the pump. One deficiency associated with these methods is the inability to precisely control the amount of cooling being accomplished. In many instances, therefore, the warm end of the pump may actually become too cold for proper performance. Therefore, auxiliary heating means may actually have to be employed in many instances. Such auxiliary heating means represents an additional and otherwise unnecessary heat load in the pump.
In addition to such efforts to prevent the conduction of heat from the warm to the cold end of cryogenic pumps, structural means are also employed in both the Riede and the Schuck patents. For example, a thin tubular section is employed to connect the cold pumping chamber to the warm packing end of the pump. Such means, however, contribute to other mechanical design problems. Pevzner, in U.S. Pat. No. 4,576,557, disclosed a unitary pump body support structure having forward and rearward pump body mounting plates and a power frame mounting plate that is precisely aligned with, and secured to, the power frame of the pump. The valve assembly, pump body, packing assembly and pump body cooling jacket can advantageously be mounted on the unitary support structure.
Other approaches taken in the pumping of a liquified compressed gas is its sub-cooling; or the use of another pump, a fore-pump, to provide an initial increase of pressure; or a refrigerant of lower temperature than the liquified gas to be pumped is applied to prevent vapor binding and, more importantly, cavitation. An example using similar means is disclosed in Japanese Patent No. 57-67773, issued Apr. 24, 1982, which describes a method to prevent cavitation of the intake liquid of a pump which receives the re-liquified gas during the delivery of a low temperature liquified gas for conversion into electricity, wherein a regenerator, a gasifier, and an expansion turbine are installed downstream from a delivery pump, and upon pressurizing, a low temperature liquified gas is delivered by the delivery pump to the downstream user. A portion of this expanded gas is routed through the above mentioned regenerator and is recondensed. This condensed liquid is then fed into a receiving reservoir and the condensed liquid in the receiving reservoir is delivered to the above mentioned gasifier by means of a condensed liquid pump. In some instances, a portion of the discharged liquid from the delivery pump, whose temperature is lower than the condensed liquid fed the receiving reservoir, is fed into the bottom of the receiving reservoir. Accordingly, the intake liquid of the condensed liquid pump is in a supercooled state in relationship to the liquid near the surface of the receiving reservoir. When such supercooled liquid is drawn in by the condensed liquid pump, the vapor pressure of the intake liquid continues to be minimal, and it is extremely unlikely that bubbling and spontaneous boiling phenomena will occur. Such a means is not applicable in the apparatus and methods of the aforementioned related patent applications, wherein the downstream use is for a liquid, in which case expanded gas for routing to the regenerator would not be available.
For large-scale industrial applications, the supply of liquid carbon dioxide is usually provided from a bulk storage system that includes a low-pressure liquid carbon dioxide tank, which is capable of delivering saturated liquid carbon dioxide to points of application under accurate temperature control. These systems are well known to those skilled in the art and they normally consist of a pumping system at the tank with insulated liquid piping in which carbon dioxide circulates past the point of application and then back to the storage tank; integral to the storage tank is a mechanical refrigeration unit. In these industrial applications when sub-cooling is desired, a mechanical refrigeration unit and heat exchanger are used to cool the liquid carbon dioxide before it reaches the point of application. In this manner, sub-cooled liquid carbon dioxide circulates continuously whether carbon dioxide is or is not being used. When the mechanical refrigeration unit and heat exchanger are located in the loop beyond the point of application, the returning liquid carbon dioxide can be cooled, resulting in a reduced refrigeration load on the bulk storage vessel. When sub cooling is incorporated in such a system specifically for prevention of cavitation, capital and operating costs constitute a deterrent to its use in many instances. Indeed, the use of liquid carbon dioxide as a refrigerant is well known. However, in the present instance, using carbon dioxide in a typical refrigeration cycle is not economically viable.
The use of consumable liquid carbon dioxide in free expansion is well known, for example, in fire extinguishers, low-temperature testing of aviation, missile, and electronic components, for pre- and post-chilling trucks, containers, railroad cars, etc., for rubber tumbling, and for controlling chemical reactions. In vacuum-insulated vessels, such as railroad cars, for example, liquid carbon dioxide is injected directly into the car wherein it expands into a mixture of solid and vapor, with temperatures from about -20.degree. F. to 50.degree. F. available from manual setting of a thermostat. Another example typical of such means is disclosed in U.S. Pat. Nos. 4,086,783 and 4,086,784, both issued May 2, 1978, which consists of an apparatus for refrigerating articles, in which a valve is utilized for selectively introducing a cryogen that is a liquid above the triple point pressure but converts to a solid and then a gas at the triple point conditions. For carbon dioxide the satisfactory operating conditions of the liquid supply was found to be about 300 psig at 0.degree. F. When the temperature in the refrigeration chamber called for more liquid carbon dioxide the valve opened and projected a conical spray of carbon dioxide which was a mixture of snow particles and gas onto deflector plates where the carbon dioxide fanned out across and longitudinally into the chamber. Complete uniform circulation of refrigerant and complete evaporation of the carbon dioxide snow was assured by fans.
FIG. 2 shows the equilibrium phase diagram for carbon dioxide (Quinn, E. L. and Jones, C. J., Carbon Dioxide, Reinhold, 1936). The figure shows the phases that carbon dioxide has at different combinations of temperature and pressure. The carbon dioxide exists as just one phase in the open spaces and the solid lines indicate where two phases exist simultaneously in equilibrium. The three equilibrium lines meet at the triple point, denoted by TP in the figure, which is the only condition at which three phases exist simultaneously in equilibrium. The triple point temperature is about -57.degree. C. (-71.degree. F.) and the triple point pressure is about 75 psia (60 psig). The equilibrium line between the liquid and vapor (gas) phases ends at the critical point, denoted by CP in the figure, where the properties of the liquid and vapor phases become identical. The carbon dioxide becomes supercritical above the critical point.
Several methods of carbon dioxide cooling are discussed in "Low-Temperature Processing in Pilot Plants" by R. L. Braun, Chemical Engineering (Jan. 16, 1978), pages 129-134. One of these methods passes liquid carbon dioxide through an expansion device wherein adiabatic direct expansion and vaporization produces a vapor-solid mixture that flows directly into the jacket of a chemical reactor. The liquid carbon dioxide is stored at 300 psig at 0.degree. F. and is expanded to snow and vapor at atmospheric pressure at -109.degree. F. Referring to FIG. 2, it can be clearly seen that when saturated liquid carbon dioxide at 300 psi and 0.degree. F. is adiabatically expanded to atmospheric pressure, the expansion follows the equilibrium curve with the existence of two phases with quality that changes from saturated liquid (at zero quality) to about 50:50 liquid and vapor carbon dioxide (quality of 0.5) as the expansion proceeds, as found when referring to a temperature-entropy diagram for carbon dioxide (not shown), until the triple point is reached wherein the two phases now become a solid and vapor phase of about the same quality. When the pressure reaches atmospheric, the temperature is now about -109.degree. F., assuming adiabatic conditions, and the carbon dioxide exhausts to the atmosphere as a vapor, with some residual crystals, or particles, of solid carbon dioxide. Problems inherent in using this type of carbon dioxide cooling means, include: plugging of the reactor jacket with solid carbon dioxide; uneven heat transfer, including localized cold spots; and poor temperature control, all of which elicit the reasons why such means are inadequate for utilization in the aforementioned inventions for the spray application of coating material diluted with supercritical carbon dioxide to achieve the state of properties essential for allowing effective spray coating onto a substrate.
It is accordingly seen that the utilization of such means for the prevention of cavitation and the suppression of undesirable phenomena associated with liquid compressibility when pumping a compressible fluid, such as carbon dioxide, are inadequate, inefficient, and costly if used with the spray coating apparatus disclosed in the aforementioned related patent and patent applications. What is needed is a low cost, direct-expansion refrigeration means, which avoids the complexities of indirect refrigeration, while avoiding the problems associated with direct adiabatic expansion of the liquid refrigerant into a vapor-solid mixture. Such a means has been found and forms the basis of the present invention.